This invention relates to improvements in an exhaust gas purifying catalyst for purifying exhaust gas discharged from a combustion device such as an engine or a boiler, and more particularly to an exhaust gas purifying catalyst for effectively removing hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx) contained in exhaust gas discharged from an automotive internal combustion engine in such a manner that NOx can be particularly effectively reduced in a lean or oxygen-excessive region of exhaust gas.
In recent years, low fuel consumption automotive vehicles have been required from the viewpoints of petroleum resource exhaustion problem and warm-up problem for the earth. In this regard, attentions have been paid on development of lean-burn automotive engines. In the lean-burn automotive engines, exhaust gas (atmosphere) becomes in a lean region in which air-fuel ratio of exhaust gas is lean as compared with a stoichiometric level, during lean-burn operation. In case that a usual thee-way catalyst is used in such a lean region of exhaust gas atmosphere, reduction of NOx can become insufficient under the effect of excessive oxygen. Consequently, it has been eagerly desired to develop exhaust gas purifying catalysts which can effectively reduce NOx even in the presence of excessive oxygen.
In view of the above, a variety of exhaust gas purifying catalysts for reducing NOx in the lean region of exhaust gas have been proposed. An example of these exhaust gas purifying catalysts is disclosed in Japanese Patent Provisional Publication No. 5-168860, in which platinum and lanthanum are carried on a porous substrate so that NOx is trapped in the lean region of exhaust gas and released when the concentration of oxygen is decreased.
However, drawbacks have been encountered in conventional exhaust gas purifying catalysts including one disclosed in Japanese Patent Provisional Publication No. 5-168860, in which they are insufficient in performance and cannot be sufficiently activated when the temperature of exhaust gas is relatively low (for example, within a low range of from 100 to 250xc2x0 C.). Additionally, sulfur is contained in fuel and lubricating oil, and therefore this sulfur is discharged in the form of oxide into exhaust gas. This sulfur poisons a NOx adsorbing or trapping material in the conventional exhaust gas purifying catalyst, thereby lowering the NOx adsorbing ability of the NOx adsorbing material.
It is an object of the present invention to provide an improved exhaust gas purifying catalyst which can effectively overcome drawbacks encountered in conventional exhaust gas purifying catalysts.
Another object of the present invention is to provide an improved exhaust gas purifying catalyst which can effectively adsorb or trap NOx in exhaust gas while can be readily released from S-poisoning.
A further object of the present invention is to provide an improved exhaust gas purifying catalyst which exhibits a high activity at low temperatures such as 150 to 500xc2x0 C.
An aspect of the present invention resides in an exhaust gas purifying catalyst comprising at least one noble metal selected from the group consisting of platinum, palladium and rhodium; and boehmite alumina. In this exhaust gas purifying catalyst, nitrogen oxides in exhaust gas from a combustion device is trapped to the exhaust gas purifying catalyst when exhaust gas is in a lean region and is reduced into nitrogen by the exhaust gas purifying catalyst when exhaust gas is in a range including a stoichiometric region and a rich region.
Another aspect of the present invention resides in a process for purifying exhaust gas, comprising: (a) preparing an exhaust gas purifying catalyst including least one noble metal selected from the group consisting of platinum, palladium and rhodium, and boehmite alumina; and (b) flowing exhaust gas from a combustion device through the exhaust gas purifying catalyst so that nitrogen oxides in exhaust gas from the combustion device is trapped to the exhaust gas purifying catalyst when exhaust gas is in a lean region and is reduced into nitrogen by the exhaust gas purifying catalyst when exhaust gas is in a range including a stoichiometric region and a rich region.